Generally, in a vehicle having an internal-combustion engine as a driving source, a lead-acid battery which supplies power to various electric loads of a starter motor or the like, is installed. The lead-acid battery is inexpensive, compared with a high performance storage battery having a high output and a high energy density of a nickel hydride storage battery, a lithium ion storage battery, or the like. However, the lead-acid battery is low in a durability of frequent charging and discharging. Especially, in the vehicles having the idle stop function (the idle reduction function), as the lead-acid battery is frequently discharged, it is considered that the lead-acid battery is early degraded. Further, in the case that the lead-acid battery is charged by a power generation of an alternator from a regenerative energy of the vehicle, as the lead-acid battery is frequently charged, it is considered that the lead-acid battery is earlier degraded. Against this, only replacing the lead-acid battery with the high performance storage battery largely increases costs.
In contrast, a vehicle battery device including a generator of an alternator or the like, a lead-acid battery which is charged with power generated the generator, and a second storage battery which is electrically connected in parallel to the lead-acid battery and is charged with power generated the generator and has a high output or energy density compared with the lead-acid battery, have been known (refer to patent literature 1). In this way, by having the second storage battery of a high performance in addition to the lead-acid battery, a degradation of the lead-acid battery is suppressed, and in addition it is inexpensive. Namely, for example, the high performance storage battery preferentially caries out power supply to the electric loads during the idle stop and a regenerative charging, and then the degradation of the lead-acid battery can be decreased. On the other hand, the inexpensive lead-acid battery preferentially carries out power supply for a long time at the time of parking the vehicle or the like, and then the high performance storage battery can be made a low capacity, and costs can be suppressed.